use command substitution for fc correction check

The check didn't work in bash v4.3.11(linux) or bash v4.3.25(osx).
Using Bash 3.2 or Zsh 5.0.6 everything worked fine.

This fixed it for me.
The check won't work in bash v4.3.11(linux) or bash v4.3.25(osx) (loaded  with "--noprofile --norc")